In a small bowl, combine the onion, garlic and salt. Cover and set aside at room temperature for 30 minutes to an hour, stirring occasionally.

Place the brie in the freezer until firm and then carefully slice off and discard the rind with a sharp knife. Cut the remaining cheese into cubes and set aside.

Cook the pasta in plenty of water until tender but firm and then drain, reserving one cup of the cooking liquid. Gently toss the cooked pasta with the onion mixture, brie, tomatoes and olive oil. Gradually add as much of the reserved liquid as needed until the brie melts and a creamy sauce coats the pasta.